1. Field of the Invention
This invention relates to gas turbine combustors and particularly to a new and improved combustor in which nitric oxide (NO.sub.x) emissions are reduced.
2. Description of the Prior Art
Undesirable pollutants, such as nitric oxide (NO.sub.x), are often produced during operation of a gas turbine engine. NO.sub.x is produced within the combustor of the engine as a result of burning of the fuel-air mixture therein. It is environmentally desirable, and, under certain governmental regulations, it is required, to reduce the amount of NO.sub.x produced to low levels.
Some combustor configurations have been found to reduce NO.sub.x emissions to acceptable levels, yet are also complex and expensive to build. For example, combustors employing water injection are effective for reducing NO.sub.x emissions, but they require holding tanks, water pumps and water supply manifolds. Two stage, lean burning combustors are also effective for reducing NO.sub.x emissions, but such combustors are also complex and expensive.
Another combustor configuration which can reduce NO.sub.x emissions is the "rich-lean" two stage combustor. The typical rich-lean combustor comprises two burning zones, a rich zone and a lean zone, separated by a quench zone. In the quench zone, air is mixed with the rich combustor gases in order to lean the gases as they enter the lean zone. By "rich" is meant that the gases have a fuel-air equivalence ratio greater than 1; by "lean" is meant that the gases have a fuel-air equivalence ratio less than 1. The rate of NO.sub.x production during burning of the combustor gases in both the rich zone and the lean zone is relatively low. However, the rate of NO.sub.x production in the quench zone, wherein the combustor gases undergo a transition from a rich to a lean condition, is relatively high. Since the NO.sub.x formation rate is time dependent, the less time that the combustor gases within a rich-lean combustor spend in the transition condition in the quench zone, the lower will be the amount of NO.sub.x produced.
Current rich-lean combustors, however, employ quenching arrangements which tend to prolong the amount of time it takes to quench the combustor gases from a rich condition to a lean condition. For example, the jet penetration distance, or distance that the quench air must travel from the quench holes in the walls of the quench zone to the center of the quench zone, is relatively large in many current combustors. The quench air must thus travel a relatively large distance to thoroughly mix with the combustor gases and, in so doing, the amount of time required to reduce the fuel-air equivalence ratio to a lean condition is extended.
Correspondingly, in order to provide jets of quench air with enough energy to travel the greater jet penetration distance, the combustor must employ large diameter, rather than small diameter, quench holes. Due to space and structural limitations, the number of quench holes which can be employed is reduced when the holes have a large diameter rather than a small diameter. The smaller number of large diameter holes are less effective for rapidly mixing the quench air with the combustor gases than would be a greater number of small diameter holes, and thus the amount of time required to reduce the fuel-air equivalence ratio of the combustor gases to a lean condition is extended.
Therefore, the greater jet penetration distance and smaller number of large diameter quench holes result in a greater amount of NO.sub.x being produced during combustion.
Some rich-lean combustor arrangements employ annular quench zones. Such an arrangement permits quench air to enter the quench zone through quench holes in both the radially inner and radially outer walls of the annulus. However, the annular height of the quench zone remains relatively large, requiring large diameter quench holes for thorough mixing. As a result, NO.sub.x production remains relatively high.
In view of the above-mentioned problems, it is therefore an object of the present invention to reduce the amount of NO.sub.x production within a rich-lean combustor by reducing the annular height of the quench zone within the combustor and thereby reducing the time required for quenching.
Another object of the present invention is to reduce the amount of NO.sub.x produced in a rich-lean combustor by increasing the number of quench holes in the quench zone of the combustor, thereby also reducing the quenching time.